In fluid processing systems in which both gas and liquid phases flow through a pipeline or conduit, slug flow or slugging can occur when a large volume of gas or of liquid known as a slug travels through the pipeline. When liquid or gas slugs exit the pipeline at a processing facility, they can be produced at a rate which exceeds the ability of the fluid handling equipment to accommodate. Slugging behavior can be categorized as hydrodynamic slugging, terrain slugging, riser slugging, or operational slugging. Combinations of these types of slugging behavior can also occur. Hydrodynamic slugging is a known multiphase flow regime that occurs at certain values of superficial gas and liquid velocities. Terrain slugging is caused by the changes in elevation in a pipeline, e.g., in the case of subsea pipeline, a pipeline along an uneven seabed with large variations in elevation. The liquid phase accumulates at a low point to form a liquid slug which then moves in the pipeline when sufficient gas pressure builds up behind it. Riser-based slugging is a special form of terrain slugging associated with pipeline risers used in offshore oil production. Liquid accumulates at a low point or a bend in the riser to form a liquid slug which then moves in the riser once sufficient gas pressure is built up behind it to push the liquid slug over the top of the riser. Operational slugging is caused by operational changes, such as start-up of a wet gas facility, and is most often handled via ramp-up of the facility.
Various schemes have been tried to control slugging behavior in fluid processing systems, including subsea fluid processing systems. The accepted control approaches to the various categories of slugging are different, because the causes of the behaviors are different, although all result in slugging behavior. For hydrodynamic slugging, the use of a “pseudo-flow” controller in which fluid flow is calculated from an equation for volumetric liquid flow through a valve is the accepted conventional approach. For terrain slugging, the accepted approach is the use of pressure control, wherein the pressure is located upstream of the slug-forming area. For riser slugging, this is at the base of the riser. For both hydrodynamic and terrain slugging, the accepted control schemes for each modulate the control valve(s) located upstream of the topside vessel(s) first receiving produced fluids (e.g., a separator, slugcatcher, free water knock-out, etc.).
Topside choke valves upstream of a vessel for first receiving produced fluids have been used in fluid processing systems to control slugging. Slug control schemes are intended to control slugging behavior by reducing the size (in terms of volume) or frequency of slugging behavior, or eliminating it entirely. Maximum production occurs with the valve fully open, but this cannot control or prevent slugging. Known control schemes include pseudo-flow control, pressure control upstream of the slug forming area, pressure control upstream of the slug forming area cascaded to pseudo-flow control, and composite variable control. Each of these has practical disadvantages. The principle disadvantage of pseudo-flow slug control is that setpoint determination is difficult. Since the pseudo-flow is not an actual physical flow rate, determination of the setpoint is not obvious. Trial and error would be required to determine such a setpoint each time it would need to be adjusted.
It would be desirable to have a mechanism for controlling hydrodynamic slugging in a fluid processing system in a way that is more practical for field use than has been realized to date.